1. Field of the Invention
The invention concerns an x-ray tube with a control end arrangement to control an electron beam that is used to generate x-rays in the x-ray tube.
2. Description of the Prior Art
In an x-ray tube, an electron beam is generated with a heatable cathode (also called a thermionic emitter), the electron beam being accelerated toward an anode serving as a target so as to generate x-rays on impact. The intensity of the generated x-ray radiation is thereby determined by the current represented by the electrons, i.e. the electrons striking the anode per time unit. Particularly in computed tomography, it can be necessary to vary the strength of the current formed by the electron beam within a few milliseconds or even microseconds.
This current typically can be controlled by means of temperature changes of the emitter. Although time constants of only a few milliseconds occur for an increase of the current, time constants of over 100 ms occur upon decreasing the current.
As an alternative to this technique, the current can be controlled by the use of a device known as a Wehnelt cylinder. Such a Wehnelt cylinder is a cylindrical control electrode that is mounted in immediate proximity to the emitter and is provided with a negative electrical potential relative to the emitter. By adjusting this potential, the number of electrons that can overcome this potential is varied, and thus the strength of the resulting current is correspondingly varied. Only relatively small currents can be controlled with a Wehnelt cylinder, however, and a significant refocusing of the electron beam by the cylinder occurs.
Grid-shaped control electrode arrangements also offer an additional arrangement for control of the beam current. Such arrangements are known from acceleration technology. A problem with such grid arrangements is that the electrons escaping from the emitter and striking the control electrodes can significantly heat said control electrodes, which can lead to the destruction of the control electrodes. Therefore, such a system operated in a pulsed manner, with the emission times of the emitter amounting to only a few percent of the total operating cycle. For example, given a pulse current of 1 A with an emission time of 1.5% and pulse frequencies in the kHz range, the average current reduces to 15 mA, which is too low for application in computed tomography, for example. Moreover, the control effect of the grid-shaped control electrode arrangement is affected by the high acceleration voltage that is present at the anode and the electrical field caused thereby. This effect of the acceleration voltage on the field caused by the control electrodes is known as the inverse field amplification factor.